Femoral component and instrumentation

ABSTRACT

A method for setting the internal-external rotational position of a prosthetic femoral component with respect to the tibial comprising: forming a cylindrical surface on the posterior femoral condyles, said cylindrical surface formed about an axis extending in a proximal distal direction with respect to the femur; placing a template having a cylindrical guide surface for engaging said posterior femoral condyles against a planar resected surface of the distal femur with said guide surfaces in contact with said cylindrically shaped posterior condyles; and rotating said template on said cylindrical guide surface as the knee joint is moved from flexion to extension until the template cylindrical guide surfaces are in a position with respect to the tibial condyle surfaces to properly balance the knee ligaments.

BACKGROUND OF THE INVENTION

The present invention relates generally to a femoral component havingcurved or radially shaped anterior surface of the posterior condyles andinstruments for use in implanting the same and more particularly forinstruments for use in balancing the ligaments extending between thefemur and tibia.

Structures that made up the knee joint include the distal femur, theproximal tibia, the patella, and the soft tissues within and surroundingthe knee joint. Four ligaments are especially important in thefunctioning of the knee—the anterior cruciate ligament, the posteriorcruciate ligament, the medial collateral ligament, and the lateralcollateral ligament. In an arthritic knee, protective cartilage at thepoint of articulation of the femur with the tibia has been worn away toallow the femur to directly contact the tibia. This bone-on-bone contactcauses significant pain and discomfort. The primary goals of a kneeprocedure are to replace the distal end of the femur, the proximal endof the tibia, and often the inner surface of the patella with prostheticparts to avoid bone-on-bone contact and provide smooth, well-alignedsurfaces for joint movement, while also creating a stable knee jointthat moves through a wide range of motion.

One of the challenges in knee surgery is to properly balance ligamenttension, especially in the medial and lateral collateral ligaments,through a full range of motion of the knee. The collateral ligaments,which connect the distal femur and proximal tibia on the medial andlateral aspects of the knee, account for much of the stability of theknee. If one of the collateral ligaments is too lax or too tightrelative to the other collateral ligament, the knee will typically beunstable, range of motion may be limited, the patella may trackimproperly, and the femur and/or tibia may wear unevenly, leading toarthritis and pain. Uneven ligament tension after knee surgery willtypically cause joint instability and poor patellar tracking, limitedrange of motion, and impaired function of the knee, as well as uneven,increased wear of the prosthetic device, which often necessitates repeatsurgery. Thus, it is imperative for the short and long-term success of aknee procedure to achieve balanced ligament tension in the knee througha full range of motion.

The components of a total knee prosthesis may be selected and positionedto balance ligament tension. Since the femoral and tibial components ofthe prosthesis are attached to cut surfaces of the distal femur andproximal tibia respectively, placement and orientation of the bone cutsare also critically important. Typically, the tibial component of theprosthesis is positioned on a flat, horizontal cut surface of theproximal tibia (at a 90 degree angle relative to the long axis of thetibia), and the position and orientation of the tibial componenttypically do not vary greatly from knee to knee. Therefore, most of thevariation in positioning of the total knee prosthesis typically occursin positioning the femoral component and the femoral bone cuts. Thesurgeon makes these femoral bone cuts to achieve a position andorientation of the femoral prosthetic component so as to optimallybalance ligament tension through a full range of motion of the knee.

SUMMARY OF THE INVENTION

Alignment of total knee replacements is achieved through severalmethods, including extramedullary, intramedullary, and computernavigation. When it comes to rotational alignment of the prosthesis inthe transverse plan (i.e. internal/external rotation) several methodsare currently utilized to achieve the “correct” rotation. They are asfollows:

a) Anatomic landmarks such as the transepicondylar axis, femoralanterior-posterior axis line and the poster condylar axis.

b) Spacer blocks or shims to balance and place the components viarotation based on the tibial resection. (see U.S. Pat. No. 4,738,254).

c) Instruments that reference the posterior/intact condyles to rotate or“jack” the femur by rotating about the medial or lateral condyle untilthe ligaments are tensioned properly (see U.S. Patent Publication US2005/0177169 A1).

The present invention helps surgeons to accomplish the correctrotational alignment by allowing the femoral trial to rotate via aradial arc created on the posterior condyles in flexion. The knee can betaken through a range of motion, and the trial will self-adjust to thecorrect rotation without the need to provide secondary mechanicalmethods to adjust or torque the knee. Gap balancing is accomplished bychanging the thickness of the insert to provide the proper balancebetween the flexion and extension space.

The femoral tibial components may be of a tricompartmental design,unicompartmental, or bicompartmental design.

In most total knee replacement systems currently available rotation ofthe femoral component about the long axis of the femur is “locked in”when the posterior condyles are resected and rotation and medial-lateraltranslation of the femoral component are adjusted independently. In thepresent invention the femoral implant and trial the rotation of thefemoral component can be adjusted after the posterior condyles areresected since the condyles are resected in an arc rather than planar.Possible disadvantages of this approach include (1) more bone may beremoved by the radial posterior cut than would be removed by a planarposterior cut and (2) rotation and medial-lateral translation of thefemoral component about the end of the femur are linked by the arc ofthe radial cut and cannot be adjusted independently.

In most total knee replacement systems, the tibial bone resection ismade at 90 degrees to the axis of the tibia. In most knees with a“normal” anatomy, if the tibial resection is at 90 degrees the femoralcomponent will need to be placed in 3 degrees of external rotation onaverage about the long axis of the femur in order for the kneereplacement system to function properly. The proper amount of externalrotation for any particular knee replacement will, of course, depend onthe individual anatomy of the patient.

Anatomical landmarks provide reference points, but are known to produceinconsistencies among patients with anatomic variations, hypoplasticdeformities or laxity of the collateral ligaments. Also, with minimallyinvasive surgical (MIS) techniques on the rise, the ability to visualizeand assess the lateral epicondyle is difficult, if not impossible. Thetransepicondylar axis is believed to be a more reproducible landmark,however, it can still introduce error when attempting to create asymmetric flexion gap. Finally, the posterior femoral axis has been usedto generally externally rotate the femoral implant 3 degrees to developa symmetric flexion gap assuming that the tibial component is cut in 90°with respect to the mechanical axis, however, it does not pay muchattention to the relationship between resection planes and soft tissuestructures during gait.

While mechanical spacer blocks or positioners allow the knee rotation tobe balanced by aligning the cutting guides with the tibial resectionwhile tensioning the ligaments, however, they do not allow for a dynamicassessment to be accomplished prior to locking in femoral rotation. Byusing a mechanism to torque the knee about either the medial or lateralpoint via a mechanical device, the surgeon could over tighten the deviceand provide too much rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded isometric view of the rotational alignment guideof the present invention;

FIG. 2 is a top view of an assembled alignment guide as shown in FIG. 1;

FIG. 3 is a partial cross-sectional view along lines 3-3 of FIG. 2;

FIG. 4 is an exploded view of a second embodiment of the presentinvention when viewed from the distal end;

FIG. 5 is an exploded isometric view of the second embodiment of thepresent invention viewed from the proximally facing side when implanted;

FIG. 6 is an assembled view of the alignment guide of the presentinvention from the distal facing side;

FIG. 7 is a cross-sectional view of the assembled second embodimentshown in FIG. 6 along lines 7-7;

FIG. 8 is an exploded isometric view of an instrument used to resect theposterior condyles in connection with the alignment guide of the presentinvention;

FIG. 9 is a distal view of an assembled instrument of FIG. 8;

FIG. 9A shows a series of possible resections of the distal femurutilizing the instrument of FIGS. 8 and 9;

FIGS. 10-17 show the utilization of the instrumentation and alignmentguide of the present invention to resect a distal femur;

FIG. 18-20 show alternate resection guides for resecting the posteriorcondyle of the distal femur utilizing the trial instrumentation of thepresent invention;

FIG. 21 is a posterior view of the femoral component to be utilized on adistal femur resected utilizing the instrumentation of the presentinvention;

FIG. 22 is a graphical layout of rotation utilized to determine theradius for the posterior condyles;

FIG. 23 is a distal view of the femur showing the normal rotational axisfor classic resection alignment;

FIG. 24 is an example of several radial positions of the radial cutsbased on the medial-lateral position of the resection guide and itsangular orientation;

FIG. 25 shows a location of a preferred radial resection; and

FIG. 26 shows the radius lowered to maintain bone stock.

DETAILED DESCRIPTION

Referring to FIGS. 1-3, there is a first preferred one-piece embodimentof a femoral trial assembly generally denoted as 120. Femoral trial 120includes a proximally facing surface 122 for contacting the resecteddistal surface of a femur. The femoral trial 120 has a pair of posteriorcondyles 124 and 126 which have curved anteriorly facing surfaces 128and 130 respectively. The curved surfaces 128 and 130 are cylindricalsurfaces each having a radius of 1.9 to 4 inches with a center locatedon or slightly offset from the center line of the distal femur. In thepreferred embodiment a pair of recesses 132 and 134 are formed in theanterior surfaces 128 and 130 to receive roller bearings 136 and 138.Bearings 136 and 138 are held within recesses 130 and 132 by pins 140and 142. Pins 140 and 142 provides the axes of rotation about whichroller bearings 136 and 138 rotate. Surface 122 has a plurality of pinholes 19 therethrough to receive bone pins for mounting the trial on thedistal femur. Two holes 18 are also provided so that a resection guidemay be pinned thereon to make the anterior or posterior chamfer cutstypically associated with a femoral component of a knee replacement.

Referring to FIG. 2, there is shown a top view of the preferred trialcomponent of FIG. 1 which shows the trial bearing surface 144 of thetrial component. Referring to FIG. 3, there is shown a partial crosssectional view showing a cross section through the posterior condylarsurfaces 124 and 126 of the trial component 120 including rollerbearings 136 and 138 mounted on pins 140 and 142 respectively.

Referring to FIGS. 4-7, there is shown an alternate embodiment of thefemoral trial in the form of a two-piece femoral trial assembly 26.Trial 26 includes a body 20 with bone pin holes 18 and 19 extendingtherethrough. Instead of rotating on bearings 136, 138 the trial body 20rotates on a plate 13 attached to the distal femur. The distal side 50of body 20 includes proximally extending posterior portions 52 and 54respectively. Rotatably mounted on body 20 is a shim plate 13 which hasa bearing element 56 which engages a slot 58 in body 20. A screw 17 isprovided to engage a threaded bore 60 of element 56 so that body 20 maybe rotated about axis 62 after the curve posterior plates 12 are engagedon a recepted posterior condyle of the femur. Other options forattaching the shim plate 13 to the trial body 20 could be rivets orwelded pins. Shim plate 13 includes a pair of slots 64 for providingclearance for the bone pins extending through bores 19 and body 20 sothat body 20 can be fixed to the bone prior to drilling locating holesthrough bore 18. Slot 64 are shaped to allow adequate rotation for thepurposes of setting internal and external rotation of the femoralcomponent. As shown in FIG. 5, shim plate 13 includes a pair ofproximally extending locking pins 14 which initially secure theassembled trial component 26 to the resected distal femur.

Referring to FIGS. 8 and 9, in the preferred method after the proximaltibia 205 and distal femur 207 are resected in a standard manner, theposterior condyles are resected by using a radial resection/sizing guidegenerally denoted as 200 which is placed onto the distal femur 207. Notethe preparation of the femur includes only making the planar distalresection. The radial resection can be centered on the distal femur, oroffset from the center line of the femur prior to making the resectionin order to account or minimize the amount of medial-lateral translationthat can occur during the trialing process as will be discussed below.One method to accomplish the offset is either placing the guide 200 offcenter, or externally rotating guide 200 prior to pinning to accomplishthe offset.

Radial resection/sizing guide 200 is comprised of a cutting blockportion 202 having a body 4 which includes a series of guide slots 204designed to guide a blade runner (a flat metal plate adapted to fitthrough the sizing guide or cutting guide slots to indicate theresection plane) to size the femur by indicating the approximate levelof the anterior resection. Body 4 also includes a guide post 5 extendingdistally therefrom as well as a series of pinholes 24 for acceptingstandard bone pins serving to attach and locate body 4 on the distalfemur. The central portion of body 4 includes three threaded hole 7, 8and 9 varying in medial-lateral position to provide different rotationalalignments, 0° (neutral), 3° left or 3° right. Obviously otherrotational alignments could be provided. An alignment guide 3 may belocated on an alignment screw 2 which is threaded into either of hole 7,8 or 9. If neutral, 3° external rotation (left knee) or 3° externalrotation (right knee) is required latch assembly 10 is used to holdguide 3 onto block portion 202. By pressing thumb latch 10′ a cam isclosed which causes the connection to be co-axial. This is inserted intohole 220 and when released will cause the cam to engage as disclosed inU.S. Publication No. 20060089641, the disclosure of which isincorporated herein by reference. A swing arm 1 which has a bore 206 forslideably receiving post 5 which is preferably cylindrical in shape andhas a threaded internal bore (not shown). Swing arm 1 can then beaxially locked onto post 5 via threaded thumb screw 6. Obviously athreads 208 of thumb screw 6 engage the internal threads of post 5.Swing arm 1 includes a saw blade guide slot 23 which guides a saw bladefor resecting the posterior condyles in a single arc as will bedescribed below. Alignment guide 3 also includes skids 22 which engagethe posterior condyles for initially locating alignment guide 202. Body4 also includes a cutout 25 which serves to allow for clearance of thepatella and other structures.

Referring to FIGS. 10 to 13, the skids 22 of guide 3 are placed underthe posterior condyles of a femur 207 to set the initialanterior-posterior location of the body 4, as is currently done withstandard alignment. Once positioned on the distal femur, the guide 202can be pinned using pin 208 using any of the holes 24 available.Alignment guide 3 in the preferred embodiment can then be removed bypressing the latch 10 and pulling guide 3 anteriorly with the kneeflexed. The swing arm 1 is then placed onto the guide post 5 and thelocking thumb screw 6 is assembled to secure the arm in place. As shownin FIGS. 12 and 13, the saw is then inserted into the slot 23 and movedin a pendulum motion to resect the medial and lateral femoral condyles.The resected posterior condyles 212, 214 are shown in FIG. 14. Byresecting the condyles individually, the cruciate ligaments can beretained.

Referring to FIG. 15, the femoral trial assembly 26 of FIGS. 4-7 is thenplaced onto the resected femur. The locking pins 14 initially willsecure the component, and a pin or screw can be inserted through thehole 15 in the shim plate 13 to provide additional stability. The tibialtrial (not shown) is then inserted onto tibia 205, and the knee can betaken through a range of motion to allow the navigation system toanalyze the alignment of the mechanical axis of the knee, or the normalflexion/extension space balancing can set the components rotation. Oncethe proper rotation is determined, the outside trial 20 can be pinnedthrough the holes 19, and the two holes 18 can then be drilled toreceive pins which locate the resection guide to make the remainingresections on the distal femur such as the anterior and anterior chamferresections (note the posterior resection does not have to be made). Thefinal step is to implant the component, which has posterior condyleshaving the same radius as the trial component 20 or 120 to completeimplantation.

FIGS. 18-20 show that the arced resection of the posterior condyles canbe accomplished in several alternate ways. An offset radial saw, or acircular slot for a burr or saw blade. FIG. 18 shows a curved slot 300mounted on body 4 of guide 202 for guiding a curved saw blade (notshown). FIGS. 19 and 20 show an arcuate cutting surface 302 which canguide a burr 304 or a curved saw blade 306 as shown in FIG. 20. Byplacing an oscillating saw like that shown in U.S. ProvisionalApplication No. 60/715,821 through the slot, a radial arc can be createdon the posterior condyles. The arcs formed are identical to that of thetrial and the prosthesis itself (FIG. 21).

The size of this arc, or the radius, is calculated to provide theminimal amount of medial/lateral femoral shift during the rotationprocess. FIG. 9A shows a layout of several different radii of curvature.The table below identifies the offset that occurs assuming thatapproximately 3° of external rotation is average, and the radius of thecut is centered on the knee joint. It is preferred to use a largerradius to remove less bone from the posterior condyles, however, thelarge shift in positioning is unwanted, so a radius is chosen that is acompromise between the two. It is important to note, that this radiusmay be optimized for sizes of the femur, either one for all, or one fora group of sizes.

TABLE I Radius ML Offset 1.901″   2 mm (.100″) 2.471″ 3.3 mm (.129″)4.00″ 5.4 mm (.209″)After the condyles are resected, the trial 20 or 120 is placed andsecured to the distal femur as described above. The knee can be takenthrough a range of motion, and the proper amount of rotation chosenbased on a functional analysis obtained from the computer algorithmsused with navigation, or simply by ligament tension in the flexedposition.

The invention allows the femoral prosthetic rotation to be assesseddynamically, without having to manually advance a mechanism in order tochange rotation. In addition, posterior osteophytes can be cleared fromthe knee, and the arthritic or damaged areas (distal and posterior) bereplaced initially with a component.

Referring to FIG. 9A, the numbers in the lower left represent threedifferent radii (approximately 1.9″, 2.5″ and 4″) for the radialresection cut on the posterior condyles. As shown in the figure, a cutalong a larger radius (e.g. 4″) removes relatively less bone, and a cutalong a smaller radius (e.g. 1.9″) removes relatively more bone. Basedon bone conservation only, a larger radius is more preferable than asmaller radius. A large radius also reduces the risk that the insertionpoints of the collateral ligaments on the femur will be affected by theradial cut. However, a large radius results in a relatively greatermedial-lateral translation for a given femoral component rotation. Theamount of M-L translation that results from 3 degrees of rotation isshown in inches in the lower middle of FIG. 9A, and also in mm in column2 of Table I. Based only on minimizing M-L translation during rotationof the femoral component, a smaller radius is more preferable than alarger radius.

The numbers in the top middle of FIG. 9A all represent the three degreesof external rotation for each of the three different radii cuts shown.The center of rotation for the 4″ radius cut is actually somewhere abovethe figure and not shown in FIG. 9A.

Note that all radial cuts are centered about the Anterior-Poster axis(aka A-P axis or Whiteside's Line).

The optimum radial cut will be one that minimizes bone loss and the riskof cutting the collateral ligament insertion points while at the sametime also minimizes M-L translation of the femoral component duringrotation of the femoral component. The optimum radius of the radial cutwill be a function of anatomy—the optimum radius for small knees will besmaller than the optimum radius for large knees. It is possible to usesurgical navigation software to help determine the optimum radius for aradial cut on any particular knee.

In the FIG. 22 the variables can be defined as:

R=the radius of the radial resection

X=the linear offset distance from the center of the circle to a pointdefined by the amount of angular offset or rotation that could occur

α=the rotation of the femoral component in degrees that could occur, and

Y=the projected medial lateral offset distance that rotation will haveon the femoral component.

The value for y or the medial-lateral offset distance that will occurduring rotation (preferably less than 3 mm) can be described dependingon different radii in the formulas below as derived from FIG. 22 asfollows:

${{\tan \; \alpha} = \frac{X}{R}};{and}$${\cos \; \alpha} = \frac{Y}{X}$ therefore:y = cos  α tan  α R  or$R = \frac{Y}{\cos \; \alpha \; \tan \; \alpha}$If  y < 3  mm  then  3 > cos  α tan  α R

Table 2 illustrates which radius values will produce various amounts ofmedial lateral shifting, depending on the amount of rotation that couldbe produced.

The goal of the radial resection knee is to provide for ability torotate femoral component when in flexion to the appropriate degree asdetermined by dynamic assessment, without dramatically shifting thecomponent medial/lateral away from the knee centerline.

TABLE 2 y y (mm) (inches) R R OFFSET OFFSET (mm) (inches) Rotation of 1degrees 0.0 0.000 0.00 0.000 0.5 0.020 28.57 1.125 1.0 0.039 57.15 2.2501.5 0.059 85.72 3.375 2.0 0.079 114.29 4.500 2.5 0.098 142.86 5.625 3.00.118 171.44 6.750 3.5 0.138 200.01 7.874 4.0 0.157 228.58 8.999 4.50.177 257.16 10.124 5.0 0.197 285.73 11.249 Rotation of 2 degrees 0.00.000 0.00 0.000 0.5 0.020 14.29 0.563 1.0 0.039 28.58 1.125 1.5 0.05942.87 1.688 2.0 0.079 57.15 2.250 2.5 0.098 71.44 2.813 3.0 0.118 85.733.375 3.5 0.138 100.02 3.938 4.0 0.157 114.31 4.500 4.5 0.177 128.605.063 5.0 0.197 142.89 5.625 Rotation of 3 degrees 0.0 0.000 0.00 0.0000.5 0.020 9.53 0.375 1.0 0.039 19.06 0.750 1.5 0.059 28.58 1.125 2.00.079 38.11 1.501 2.5 0.098 47.64 1.876 3.0 0.118 57.17 2.251 3.5 0.13866.70 2.626 4.0 0.157 76.23 3.001 4.5 0.177 85.75 3.376 5.0 0.197 95.283.751

This is accomplished by aligning the radial resection close to thenormal rotation of the femur so that the component rotation based on thecentral axis of the radial cut is closely approximated to the finalposition (assuming that the “normal” rotation is 30 external). FIG. 23shows the normal rotation of the femoral component where the AP axis isperpendicular to the epicondylar axis, and final femoral componentimplantation would be such that the central plane of the femoralcomponent in the saggital plane is in line or parallel with the AP axis;FIG. 24 shows the effect of not centering the radial cut (cutting guide)on the knee. It shows three curves A depicting radial cuts based onmedial-lateral position of the resection guide and angular orientation.

A radius value is chosen and the location on the femoral component forthe radial cut which provides for both a minimal amount of offset duringrotation and minimal amount of posterior condylar bone removal. Theprepared maximum amount of tolerable or justifiable values are: rotationshould be 3° and the maximum amount of resultant medial-offset shouldless than or equal to 3 mm, the radial value from Table 2 above is2.251″. To satisfy the minimal amount of bone removal, the position ofthe radial resection in the anterior posterior direction (distance fromthe posterior condylar plane) can be adjusted to achieve the designgoal.

FIG. 25 shows a radial resection, location A using a standard femoralimplant. Current implants have a flat posterior resection level B. Byplacing the radius on the component, it can easily be seen that thereare regions of bone that will be removed from the femur to accommodatethe radius. This figure shows an increased amount of bone C overstandard (planar resection B) that would be removed.

FIG. 26 shows the radius lowered to a point where the above minimal boneremoval goal is achieved, while maintaining a radius that willaccommodate the rotation limits (up to 3° beyond the “normal” bothinternal and external rotation) and no greater than 3 mm ofmedial-lateral offset at the maximum of 3° additional rotation. Thefactors that allow this curve to be lowered, is that the curve will passthrough points in the femur that define the condylar width W. Condylarwidth W is further defined as the minimal dimension for a particularfemoral component that will maintain the same contact stress levels(area) as originally designed. This defines the center location of theguide post 5 which takes into account for implant design. It movescenter point based on distance E-F and G-H. Thus the posterior radiusgoes through points E and H which minimize the amount of bone removedwithout reducing load bearing condylar features.

In summary, the location of the radial resection in the medial lateralplane should be located approximately in the location of the center ofthe knee and parallel to the AP axis of the femur, and its height in theanterior posterior direction is derived from the posterior condylarplane a distance that will maintain the condylar width of the implantdesign.

The optimum amount of rotation of a femoral component for a particularknee is determined through the use of the knee trials 20, 120 that havebeen designed to also rotate about the radial cuts made on the posteriorcondyles.

Referring to FIGS. 16 and 17, there is shown a typical femoral cuttingblock generally denoted as 150 mounted on a resected distal femur 207 bymounting pins 152 as is typical resection guide 150 has a cutting guideslot 156 for making a posterior chamfer cut on the distal femur and aguide surface 158 for making an anterior cut on the distal femur. Acutting guide surface 160 is provided for making an anterior chamfer cuton the distal femoral surface. As shown in FIG. 17, both the resectedfemur 207 and tibia 205 are shown fully prepared to receive a prostheticimplant on both the femur and tibia.

Referring to FIG. 21, there is a typical femoral implant generallydenoted as 250 viewed from its proximal bone contacting side 252 showingthe two condylar portions 254 and 256 with the rounded posteriorcondylar bone contacting surfaces 258 and 260 respectively. As istypical, a pair of bone engaging pins 262 and 264 are provided on theimplant 250 for engaging the distal surface of the prepared femur 207.Femoral component 250 may be either attached to the resected distalfemur with bone cement or as a press-fit prosthesis using tissueingrowth.

While a posterior femoral referencing system is disclosed above oneskilled in the art would understand that an anterior referencing systemcould be utilized.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for setting the internal-external rotational position of aprosthetic femoral component with respect to the tibial comprising:forming a cylindrical surface on the posterior femoral condyles, saidcylindrical surface formed about an axis extending in a proximal distaldirection with respect to the femur; placing a template having acylindrical guide surface for engaging said posterior femoral condylesagainst a planar resected surface of the distal femur with said guidesurfaces in contact with said cylindrically shaped posterior condyles;and rotating said template on said cylindrical guide surface as the kneejoint is moved between flexion and extension until the templatecylindrical guide surfaces are in a position with respect to the tibialcondyle surfaces to properly balance the knee ligaments.
 2. The methodas set forth in claim 1 further comprising mounting said template to theresected surface of the distal femur with a first mounting elementlocated on said distal femur planar surface at an intersection with saidaxis.
 3. The method as set forth in claim 2 further comprising holdingsaid template in a selected position with a second mounting element. 4.The method as set forth in claim 3 wherein said first mounting elementis a bone screw and said second mounting element is a bone pin.
 5. Themethod as set froth in claim 1 wherein said template cylindrical guidesurface has an external shape conforming to the shape of the condyles ofa prosthetic femoral component.
 6. The method as set forth in claim 1wherein said cylindrical guide surface of said template has a rollerbearing mounted in a recess therein.
 7. An instrument for setting theinternal-external rotation of a prosthetic femoral component withrespect to a proximal tibia comprising: a body having a planar surfacefor engaging a planar surface of a resected distal femur and a cuttingguide for forming a cylindrical surface on a femur coupled to saidplanar surfaces, said cylindrical surface defined by an axis extendingin a direction generally perpendicular to said planar surface.
 8. Theinstrument as set forth in claim 7 wherein said planar body surface hasa central pivot means for allowing rotation of said cutting guide withrespect to the distal femur about said axis.
 9. The instrument as setforth in claim 8 wherein the pivot means is a first aperture in saidplanar body surface allowing rotation of the cutting guide about a pivotelement engaging the femur.
 10. The instrument as set forth in claim 9wherein said pivot element comprises a bone screw for engaging the boneof the distal femur said screw having a bearing surface thereon forrotatably engaging the aperture.
 11. The instrument as set forth inclaim 9 wherein said aperture is an oblong slot.
 12. The instrument asset forth in claim 11 wherein the oblong slot has a long axis in amedial-lateral direction.
 13. The instrument as set forth in claim 7wherein said planar body surface further comprises a second aperturespaced from said first aperture for receiving a locking pin for fixingthe rotational position of said body on the femur.
 14. The instrument asset forth in claim 7 wherein said cylindrical surface has a first andsecond part for engaging cylindrical posterior surfaces of the medialand lateral femoral condyle.
 15. The instrument as set forth in claim 14wherein said cylindrical surface includes at least one roller bearingfor engaging the posterior surfaces.
 16. A prosthetic femoral componentfor use on a prepared distal femur comprising: a distal portion; and atleast one posterior femoral condylar portion coupled to said distalportion having an inner bone-contacting surface and an external bearingsurface, said inner bone-contacting surface having a cylindrical shapewith the central axis of the cylinder located within the intercondylarnotch area of the distal femur and extending in a proximal-distaldirection with respect to the distal femur.
 17. The prosthetic femoralcomponent as set forth in claim 16 wherein said axis is approximatelyparallel with the mechanical axis of the femur.
 18. The prostheticfemoral component as set forth in claim 16 wherein the component has twoposterior condylar portions.
 19. The prosthetic femoral component as setforth in claim 16 further comprising at least one anterior condylarportion coupled to said distal portion.
 20. The prosthetic femoralcomponent as set forth in claim 19 wherein the component has twoanterior and posterior condylar portions.
 21. The prosthetic femoralcomponent as set forth in claim 16 wherein the medial side of eachfemoral condyle is thinner than the lateral side of each condyle.